The invention relates to the field of mode couplers used in receivers, and in particular to mode discriminating for an optical receiver connected to dual mode/multimode fibers.
Most fiber links greater than 200–300 m use lasers with the wavelengths greater than 1200 nm and specifically at the wavelengths around 1310 nm, 1490 nm, 1510–1570 nm, and 1620 nm. On the other hand, fiber links less than 200 m are dominated by a wavelength around 850 nm. The standard fiber for use in the 1200–1600 nm range have core diameter of approximately 8 μm and many of these fibers become dual mode for 850 nm transmission.
Use of standard fibers (i.e., those that are designed to be single mode in the 1200–1600 nm window) with 850 nm wavelength would result in excitation of both the modes (with variable amplitudes depending on the launch conditions as well as factors such as the imperfections and bends in the fiber). These modes travel at different speeds and thus high speed transmission becomes difficult due to overlap of data pulses after traveling some distance in the fiber.
At the other extreme, multimode fibers are used with a typical core diameter of 50/62.5 micron that support 100's of modes. Each of these modes travel at different speeds and the resulting modal dispersion limits the bandwidth-distance product. Recently, many have used restricted mode launch, e.g., launch laser beam in the center of the fiber that significantly underfills the multimode fiber core to increase the effective bandwidth-distance product due to excitation of fewer modes.